1. An individual that is considered to be a newbie (newb), noobie (noob), or nub.
2. An individual that is new to something.
3. An individual that is not very good at something.
The new guy is a nubski, he has no idea what he's doing.
The foreskin of a nub (or noob, a person who is new and/or plays a game childlike.) Calling someone a nubskin suggests that they play so poorly, there are indeed the foreskin of the nub.